I have noted this on several of the games. The computer builds for itself more tanks than is possible within the time. Eg start World 4671 on 40*40 with standard piece build rates, 1 human and 5 computer opponents. Start off with a parachutist. Head NE for two squares and then SE. The island with pink opponents and only one city shows 3 tanks by day 8 - impossible.

The only way I could make the "Clever" level stand a chance of beating a good player was to make the computer build a bit quicker. "Medium" builds at the same rate as you so you could consider that a fair opponent, and "Stupid" builds more slowly than you.